Many devices used in electronics such as lead frames, printed circuit boards and connectors are often subjected to silver plating followed by soldering. The silver plating, however, causes many problems such as discoloration of silver-plated parts, formation of copper oxide layer, poor adhesion in molding and water pollution problem.
To ameliorate such problems, therefore, various palladium or palladium alloy plating compositions have been developed. The use of a palladium or palladium alloy is attractive due to its ability to enhance the electric conductivity and its excellent corrosion resistance, abrasion resistance and non-porosity to various chemicals and substances. In a palladium or palladium alloy plating composition, however, it is important that the palladium and other metal compounds are provided in a stable state, and a complexing agent and a conductive salt are properly selected depending on the metal components so as for the composition to have a good stability and uniform platability.
For instance, U.S. Pat. No. 4,741,818 discloses a palladium or palladium alloy plating electrolyte comprising a palladium compound, one or more alloying metal compounds and a complexing agent such as chelidamic acid, orotic acid or 2-pyrrolidone-5-carboxylic acid, in order to improve the stability and platability.
U.S. Pat. No. 4,715,935 discloses a method for plating a substrate with a palladium alloy plating composition comprising a palladium source, an oxalate as a complexing agent and, optionally, an additional alloying metal compound.
However, the above compositions still suffer from unsatisfactory solderability and low flexibility when applied on a lead frame and the like due to, e.g., an increase in the internal stress caused by hydrogen brittleness; and, therefore, a need has still existed for a stable plating composition containing palladium with improved soldering characteristics.